(CNN) -- When Leidy Sanchez and her husband, Carlos Reyes, went to the hospital last week to deliver their baby, a nurse got her a gown, hooked her up to a fetal monitor and asked an unexpected question: Would they like to donate cells from their baby's umbilical cord blood to a public bank?
"We said, 'For real, people are doing this?' " says Sanchez. "We'd never heard of it."
The couple had heard of private banking, where you pay thousands of dollars to store your baby's cord blood cells, but this was different. The stem cells from Christopher's umbilical cord would be stored, free of charge, at a public bank for potentially anyone in need of a stem cell transplant for leukemia, sickle cell anemia or dozens of other diseases.
It didn't take Sanchez and Reyes long to say yes.
"We felt good that we could save a life," Sanchez says.
But what about if, heaven forbid, their baby Christopher were to need those cells later in life? They might still be there for him, but then again they might have been given away to someone else.
"I don't think he'll need them. Hopefully, he'll be healthy," Sanchez says. "But we know there could be someone out there who needs these cells right now."
While private banks advertise heavily to parents, public banks do not, and they hope stories such as Christopher's will encourage other families to donate.
"If people donated to public banks instead of private ones, many, many lives could be saved," says Dr. Andromachi Scaradavou, medical director of the National Cord Blood Program. "The numbers are huge."
But public banks know they face an uphill battle: Parents will always wonder if they should keep those cells for themselves.
Umbilical cord cells often useless to the donor
Transplant specialists, and even private banks themselves, say umbilical cord cells are often of no use to the child who donated them.
For example, if a child develops leukemia, there are usually leukemia cells in the cord blood, making them inappropriate for a transplant, transplant experts say. Or if a child has a genetic disorder, such as sickle cell, that same problematic DNA lies in the cord blood cells, rendering them useless.
"I've treated thousands of patients, and I've never used a child's own cord blood cells to treat leukemia, or sickle cell, or other genetic diseases, or immune deficiencies, or bone marrow failure," says Dr. Joanne Kurtzberg, director of the pediatric blood and marrow transplant program at Duke University Medical Center.
Other transplant specialists agree.
"When a kid comes in with leukemia and the mom says, 'We stored his cord blood cells,' I've never used it," says Dr. Haydar Frangoul, director of the pediatric bone and marrow transplant program at Vanderbilt University Medical Center.
"It would be ridiculous to use the child's own cells," he adds. "It would be like taking someone with cystic fibrosis who needs a lung transplant and giving them back their own diseased lung."
Scaradavou, a consultant at the pediatric bone marrow transplant program at Memorial Sloan-Kettering Cancer Center, agrees.
"There are very, very, very few situations where a kid could benefit from his own cord blood," she says. "It would be like winning the lottery."
Kurtzberg says she is using children's own cord blood cells experimentally to see if they can help treat cerebral palsy or brain damage caused by low oxygen at birth.
"We're in the middle of our study, so we don't know yet if it works," she says.
Kathy Engle, a spokeswoman for Cord Blood Registry, a private bank, says it's true that a child's own cells often won't work for himself, but that sometimes they do.
"All transplant physicians make their own determinations on the best route for treatment," she says.
In the future, there could be even more uses for umbilical cord cells, she adds.
"Preserving cells today could be something that has very different uses in the future," Engle wrote in an e-mail. "Of course, we cannot predict that, but unfortunately there is only the one chance to collect the cord blood, so parents' decisions are time bound."
Umbilical cord cells helpful for siblings
A child's umbilical cord cells could be useful for a sibling or other family member who needs a transplant, doctors say.
"When I have a patient whose mother is pregnant, I say they absolutely should go ahead and privately bank those cord blood cells when the baby's born," Frangoul says.
But he adds that often umbilical cord cells aren't enough and doctors have to go in and extract marrow from the child who donated the umbilical cord cells to use in addition.
"Every time I've used cord blood, I've had to go in and get marrow, too," Frangoul says.
He says that's why he chose not to privately bank his own children's umbilical cord cells.
"My wife got all these brochures in the mail, and said, 'Are you sure we don't want to do this?' " he remembers. "But I said, 'Look, it's not going to help the baby, and if we need his cells for a sibling, he'll always be here. He's available. We can always go in and get his marrow."
Private banks advertising too aggressively?
Frangoul, whose sons are 9 and 11, remembers having to do quite a bit of work to persuade his wife not to privately bank their children's umbilical cord cells.
"These companies advertise heavily and really make the moms feel guilty if they don't bank the cells," he says. "I think it's a travesty."
The Frangouls publicly banked their older son's cells, but they didn't for their younger son since the option wasn't available at the hospital where they delivered.
Private cord blood banks defend their marketing to families.
"Our company does not condone the use of guilt or fear-based marketing tactics," says Scott Brown, a spokesman for FamilyCord, a private bank. "FamilyCord advertising is focused on educating families on the current treatments and future potential of stem cells."
If I do public banking, will the cells be there for my family?
If you choose to donate to a public bank, the cells might still be there should you need them.
According to Kurtzberg, director of the Carolinas Cord Blood Bank at Duke University Medical Center, there's a 95 percent chance your child's umbilical cord cells will still be there if you need them, and you get to have them for free.
Here's the catch: Public banks have strict standards and reject about half of all donations because not enough cells were obtained or there are quality problems with the cells, Kurtzberg says. In that case, you can't get your child's cells back because they weren't stored in the first place.
How to publicly bank your child's umbilical cord cells
Here's a list of some 200 hospitals that make it easy to publicly donate your baby's umbilical cord cells. Each hospital has a relationship with one of the more than 40 public banks in the United States.
If you're not delivering at one of these hospitals, the National Marrow Donor Program says you can contact a public bank directly about arranging a donation.
A mother can't donate her baby's cord blood if she has certain infections such as HIV or if she has tattoos. Here's a complete list of eligibility requirements.
CNN's John Bonifield, Sabriya Rice and Jennifer Bixler contributed to this report.